By-pass system

ABSTRACT

A method and apparatus providing positive control of steam conditions during start up, shutdown and low load operation of the vapor generator including by-passing vapor around the heating, reheating and turbine zones for discharge to the condenser and using the by-passed vapor to attemperate the outflow from the heating and reheating zones.

United States Patent 1191 Durrant et a1.

[ May 13, 1975 I BY-PASS SYSTEM [75] inventors: Oliver W. Durrant; Kurt l-l. l-laller,

both of Akron, Ohio [73] Assignee'. The Babcock & Wilcox Company, New York, NY.

221 Filed: Apr. 18,1972

21 Appl.No.:245,2l4

[52] U.S. CI. 60/646; 60/662; 60/666; 60/677; 60/679 [51] Int. Cl. F0lk 13/02 [58] Field of Search 122/429 R, 406 R, 406 ST; 60/645, 646, 670, 677, 679, 666, 663, 662

[56] References Cited UNITED STATES PATENTS 2,968,156 1/1961 Pacault et a1. 60/73 3,021,824 2/1962 Profos 122/406 S 3,100,967 8/1963 Brunner 60/105 X 3,175,367 3/1965 Gorzegno et a1. 60/73 3,219,018 11/1965 Augsburger 122/406 ST 3,243,961 5/1966 Caracristi 60/105 3,250,259 5/1966 Profos 122/406 S FOREIGN PATENTS OR APPLICATIONS 821,790 7/1949 Germany 60/73 513,869 9/1952 Belgium 60/104 547,668 10/1956 Belgium 60/104 891,323 9/1959 United Kingdom 60/105 Primary E.\'aminer--Edgar W. Geoghegan Assistant ExaminerAllen M. Ostrager Attorney, Agent, or Firm-Joseph M. Maguire; Robert J. Edwards [57] ABSTRACT A method and apparatus providing positive control of steam conditions during start up, shutdown and low load operation of the vapor generator including bypassing vapor around the heating, reheating and turbine zones for discharge to the condenser and using the by-passed vapor to attemperate the outflow from the heating and reheating zones.

4 Claims, 3 Drawing Figures PATENTED HAY 1 31975 3,882,680

UL 5 J nv-rxss SYSTEM BACKGROUND OF THE INVENTION The invention relates generally to fossil-fuel fired power plant systems and more particularly to a by-pass arrangement providing positive control of steam conditions during cold and hot starts, shutdown and low load operation.

As more and more nuclear units are placed in service with low incremental loading cost, more fossil-fuel fired units will be used to make up the fluctuating portion of a system demand. Many of the latter type units now being ordered are intended for initial or future cyclic loading including such requirements as nightly and weekend shutdowns, with rapid reloading on weekday mornings. This type of operation means that the boiler and turbine will be subjected to many start ups and shutdown cycles during their operating life. The cycling of the load itself is of relatively minor consequence unless it produces temperature differentials which generate thermal stresses, thus the temperature differences between adjacent components and through the thickness of a part must be considered.

The present state of the art does not provide effective means for controlling boiler outlet superheat and reheat steam temperatures during start up, shutdown and low load operation. The control of steam temperature characteristic is particularly critical with respect to the turbine and associated components where the need for matching steam and metal temperatures has long been recognized. The means for controlling superheat and reheat steam temperatures at normal operating loads, such as water attemperators at the secondary superheater and reheater inlets, are not effective for start up or low load operation. During these phases of operation, the superheat and reheat steam temperatures approach the flue gas temperature as a result of the disproportionately high ratio of heating surface and gas flow to steam flow. Moreover, the present procedures for achieving matched temperatures, such as hold periods at low load operation and protracted start ups, are not suitable for cyclic service.

SUMMARY OF THE INVENTION The present invention is directed at a by-pass system which provides an arrangement for positive control of steam conditions with the concomitant result of faster start ups, controlled shutdowns in preparation for anticipated start up, reliability and increased maneuverability during start up and over the load range, and conservation of equipment life.

The present invention achieves positive control of steam conditions through the control of drum pressure by means of a superheater by-pass to the condenser, the control of superheat steam temperature by means or" a superheater outlet steam attemperator and a superheater stop valve and stop valve by-pass between the primary and secondary superheaters, and the control of reheat outlet steam temperature by means of a reheat outlet steam attemperator.

BRIEF DESCRIPTION OF THE DRAWINGS FiG. l is a diagrammatic illustration of a typical steam system for a fossil-fuel fired power plant embodying the invention.

FIG. 2 is a detail view of the steam attemperating apparatus embodied in the invention.

FIG. 3 is a detail view taken along line 3-3 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is illustrated in diagrammatic form a vapor generator 10 comprising burner means 12 arranged to supply high temperature combustion products and an outlet means 14 for discharging the exhaust flue gases. The fluid circuitry of the vapor generator I0 has a series flow path including an economizer 16, a steam-water drum 18, a downcomer circuit 20, a steam generating section 22, a riser circuit 24, a primary superheater 26, a secondary superheater 28 and a reheater 30.

Feedwater is supplied to the vapor generator 10 by a high pressure feed pump 32 and is preheated as it passes through the economizer 16. The preheated feedwater is conveyed by conduit 34 and fed into steamwater drum 18 to be recirculated therethrough and through the steam generating section 22 by the downcomer and riser circuits 20 and 24, respectively. The drum 18 includes a separating section 36 for removing the steam from the steam-water mixture leaving the riser circuit 24 and conveying the saturated steam by a conduit 38 for passage through the primary superheater 26, wherein initial superheating of the steam takes place, and then by a conduit 40 for passage through a water spray type attemperator 48 and through the secondary superheater 28, wherein final superheating of the steam is achieved. In accordance with the invention, the conduit 40 contains a stop valve 42 and is flow-connected with a stop valve by-pass conduit 44 which contains a control valve 46. The superheated steam leaving the secondary superheater outlet header 50 is conveyed by a conduit 52 to the high pressure turbine 54 for partial expansion therein. The partially expanded steam leaving the high pressure turbine 54 is conveyed by a conduit 56 through a water spray type attemperator 58 for passage through the reheater 30, wherein reheating of the partially expanded steam takes place. The reheated steam leaving the reheater outlet header 60 is conveyed by a conduit 62 to the low pressure turbine 64 for final expansion therein. The exhaust steam from the low pressure turbine 64 is conveyed by a conduit 66 to a main condenser 68 where it is condensed under vacuum for return to the feedwater system.

In accordance with this invention, a by-pass system is provided around the primary and secondary superheaters, the reheater, and the high and low pressure turbines. The by-pass system is constructed and arranged to provide positive control of drum pressure and superheat and reheat outlet temperatures during hot and cold start ups, for low load operation, and during normal or emergency shutdown of the vapor generator.

The by-pass comprises a conduit 70 containing a stop valve 72 and a control valve 74 and having its inlet end connected and opening to conduit 38 intermediate of the drum 18 and the primary superheater 26 and its discharge end connected and opening to the main condenser 68 which is arranged to receive and condense the saturated steam for return to the feedwater system. It will be understood, that the inlet end of the by-pass system may alternatively be flow-connected directly to the drum 18.

In accordance with the invention, provisions are made for diverting saturated steam flow from the bypass conduit 70 to a position downstream flow-wise of the secondary superheater 28 by means of a conduit 76 containing a control valve 78, and a stop and check valve 80, and having its inlet end connected and opening to the by-pass conduit70 downstream flow-wise of the stop valve 72 and its discharge end flowconnected to a steam distribution manifold 82 disposed within the secondary superheater outlet header 50. Separate provisions are made for diverting saturated steam flow from the by-pass conduit 70 to a position downstream flow-wise of the reheater 30 by means of a conduit 84 containing a control valve 86, and a stop and check valve 88, and having its inlet end connected and opening to the by-pass conduit 70 downstream flow-wise of the stop valve 72 and the inlet to conduit 76, and its dis charge end flow-connected to a steam distribution manifold 90 disposed within the reheater outlet header 60. Further provisions are made for diverting saturated steam to the turbine seals by means of a conduit 92 containing a control valve 94.

It will be understood that the discharge ends associated with the conduits 76 and 84 may alternatively be flow-connected to any suitable type steam attemperator disposed intermediate of the secondary superheater 28 and the high pressure turbine 54, and intermediate of the reheater 30 and the low pressure turbine 64, in conduits 52 and 62 respectively.

Referring to FIGS. 2 and 3 there is illustratedrdetail views of the steam attemperating apparatus in accor dance with the invention and including the manifold 82 containing a plurality of spray holes 96 equally spaced across the manifold 82 and facing the incoming steam flow being discharged from the secondary superheater outlet legs 98 and into the secondary superheater outlet header 50, the manifold being centered within the header 50 by supports 100. The attemperated steam is discharged from header 50 through the conduit 52. It will be understood that the steam attemperating apparatus associated with the reheater outlet header 60 is substantially of the same construction as abovedescribed.

During the typical cold start up or a start following a weekend shutdown, the requirements of firstly, a low steam temperature for good steam to turbine metal temperature matching and secondly, a high heat input for a quick start up, are generally not compatible. The steam flow during start up is relatively low (up to about to percent of maximum flow), thereby resulting in a superheat outlet steam temperature closely approaching the gas temperature in the vicinity of the superheater outlet legs 88. In order to reduce steam tem-.

perature for matching purposes, the prior art has resorted to lowering the heat input to the vapor generator, however, this has not always been possible to the desired degree since heat input must be maintained high enough to generate sufficient steam for rolling and initial loading of the turbine. In accordance with the invention, the superheater outlet steam attemperator as represented by distribution manifold 82 utilizes saturated steam to overcome this problem by permitting steam temperature control independent of heat input to the boiler. In order to by-pass saturated steam from the drum 18 through the conduit 76 to the secondary superheater outlet header 50, the stop valve 42 is closed and the stop valve by-pass control valve 46 is regulated to provide the required flow resistance 7 through the superheater and to control steamflow therethrough. In addition to providing positive control of steam temperature and .a reduction. in start up time, this arrangement furnishes the. means for dual pressure operation of the vapor generator wherein the'turbine 54 may be started at a low throttle pressure while maintaining drum pressure sufficiently high to satisfy the.

steam seal system being supplied through conduit 92.

During the typical cold start up or a start following a weekend shutdown and before steam is admitted to' the highpressure turbine 54, the reheater iswithout steam flow, thus the reheater metal absorbs heat from the flue gas and eventually attains the temperature. level of the flue gas. Subsequently, when steam is first.

admitted to the turbine 54 and passes through the re heater 30, it quickly rises to the gas temperature level, 2 resulting in a poor match with the relatively low' metal temperatures of the low pressure turbine 64. In accordance with the invention, the steamleaving the re heater 30 is attemperated with saturated steam delivered by the conduit 84 through the spray attemperator, I v as represented by distribution manifold 90 disposed within the reheater outlet header 60, thereby providing a positive reheat steam temperature control during the start up.

During the typical hot start, including a start follow-d ing an overnight shutdown, the turbine metal temperatures are high, thus the heat input to the vapor generaa tor must be sufficiently high to provide the superheat and reheat steam temperatures required for matching the steam and metal temperatures. The higher heat input generally causes too rapid a rise in turbine throtand thereby provide the means for holdingthe throttle steam pressure at whatever intermediate valueis required to allow turbine steam admission through wide open stop valve by-pass valve and control valves thus;

eliminating throttling temperature drops.

During the typical shutdown and in anticipation of an early re-start, heat may be retained in the superheater outlet and the main steam leads, bydepressurizing the. vapor generator through the by-pass conduit to the) main condenser 68 or, alternatively, by regulating the stop valve 42 and stop valve by-pass valve 46 to keep the boiler at full pressure and to reduce the steam pressure to the turbine while maintainingfull temperature 3 thereto. Both modes of shutdown maintain a close match between turbine metal and outlet steam temperatures.

While in accordance with the provisions of the fstat- 1 utes there is illustrated and described herein a specif c embodiment of the invention, those skilled in the art. will understand that changes may be made in the form' j of the invention covered by the claims, and that certain features of the invention may sometimes be used toadvantage withouta corresponding use of the other fea- H tures.

What is claimed is:

1. In a power plant system having vapor generating means, turbine means flow connected therewith, vapor superheating means flow connectedly interposed 'between the generating and turbine means, and condenser means disposed in flow receiving relation with the turbine means. the improvement comprising conduit means by-passing said superheating and turbine means, valve means for adjusting the vapor flow through the superheating and conduit means, said conduit means including means for admixing at least a portion of the by-passed vapor with the superheated vapor to maintain the vapor temperature entering the turbine means within a given limit, and means for discharging the remaining by-passed vapor into the condenser means.

2. in a power plant system according to claim 1 wherein said turbine means includes a high and a low pressure turbine, vapor reheating means flow connectedly interposed between said high and low pressure turbines, said conduit means including means for admixing at least a portion of the by-passed vapor with the reheated vapor to maintain the vapor temperature entering the low pressure turbine within a given limit.

3. The method of starting and shutting a power plant system in which, during normal operation, a vaporizable fluid is successively passed through vapor generating means, vapor superheating means, turbine means and condenser means, and including conduit means bypassing the superheating and turbine means, and comprising the steps of passing at least a portion of the generated vapor through the conduit means, admixing at least a portion of the by-passed vapor with the superheated vapor to maintain the vapor temperature entering the turbine means within a given limit, and discharging the remaining by-passed vapor to said condenser means.

4. The method of starting and shutting a power plant system according to claim 3 wherein said turbine means includes a high and a low pressure turbine, vapor reheating means flow connectedly interposed between the high and low pressure turbines, and including the step of admixing at least a portion of the by-passed vapor with the reheated vapor to maintain the vapor temperature entering the low pressure turbine within a given limit. 

1. In a power plant system having vapor generating means, turbine means flow connected therewith, vapor superheating means flow connectedly interposed between the generating and turbine means, and condenser means disposed in flow receiving relation with the turbine means, the improvement comprising conduit means by-passing said superheating and turbine means, valve means for adjusting the vapor flow through the superheating and conduit means, said conduit means including means for admixing at least a portion of the by-passed vapor with the superheated vapor to maintain the vapor temperature entering the turbine means within a given limit, and means for discharging the remaining by-passed vapor into the condenser means.
 2. In a power plant system according to claim 1 wherein said turbine means includes a high and a low pressure turbine, vapor reheating means flow connectedly interposed between said high and low pressure turbines, said conduit means including means for admixing at least a portion of the by-passed vapor with the reheated vapor to maintain the vapor temperature entering the low pressure turbine within a given limit.
 3. The method of starting and shutting a power plant system in which, during normal operation, a vaporizable fluid is successively passed through vapor generating means, vapor superheating means, turbine means and condenser means, and including conduit means by-passing the superheating and turbine means, and comprising the steps of passing at least a portion of the generated vapor through the conduit means, admixing at least a portion of the by-passed vapor with the superheated vapor to maintain the vapor tempErature entering the turbine means within a given limit, and discharging the remaining by-passed vapor to said condenser means.
 4. The method of starting and shutting a power plant system according to claim 3 wherein said turbine means includes a high and a low pressure turbine, vapor reheating means flow connectedly interposed between the high and low pressure turbines, and including the step of admixing at least a portion of the by-passed vapor with the reheated vapor to maintain the vapor temperature entering the low pressure turbine within a given limit. 